Several ambitious programs have been initiated to develop the next generation of advanced aircraft for aerospace transportation. It is the goal of programs such as the Integrated High Performance Turbine Engine Technology (IHPTET) program, the National Aerospace Plane (NASP), and the High Speed Civil Transport (HSCT) to develop turbopropulsion capabilities far beyond those achieved using today's technology. To reach these goals structural materials are needed that can withstand extreme temperatures and hostile environmental conditions.
Advanced high temperature materials are the key to successfully developing the next generation aerospace propulsion and power systems. Advanced materials will enhance the performance of these systems by allowing higher speeds, expanded flight ranges, and increased payload capabilities. The materials must be light weight and able to reliably withstand the stresses associated with flight at speeds in excess of Mach 2.5. Due to increased temperature capabilities and relatively low densities, composite materials are widely recognized as the advanced materials needed for future systems.
A principle obstacle to the development of such systems, is the lack of high temperature composite materials having a combination of high specific strength, fracture toughness, high and low temperature oxidation resistance and high temperature creep resistance. To date, only nickel aluminide and molybdenum disilicide appear to have the potential for such high temperature applications as the HSCT exhaust nozzle because most other intermetallic systems exhibit detrimental high temperature oxidation behavior. Of these, molybdenum disilicide appears to be the most favorable candidate because it is a silica former and can withstand temperatures up to 1773K, roughly 473K higher than nickel aluminide which is an alumina former.
The intermetallic compound molybdenum disilicide (MoSi.sub.2) has long been known to have excellent high temperature oxidation resistance. It has the additional advantages of low cost, a high melting point of 2300K, a relatively low density of 6.5 gm/cc (versus 8 gm/cc for current engine materials), it is non-toxic and environmentally benign, and can be easily machined using standard metallurgical practices. All of these advantageous properties make molybdenum disilicide an extremely attractive structural material for high temperature aerospace applications.
Unfortunately, the successful use of MoSi.sub.2 has been hindered due to its brittle nature at low temperatures, relatively high coefficient of thermal expansion, average creep resistance at high temperatures and, most importantly, its accelerated oxidation at temperatures between about 673 and 773K. The accelerated oxidation of MoSi.sub.2 at intermediate temperatures causes the material to disintegrate into powder, a phenomenon known as pesting. Thus, while MoSi.sub.2 will perform exceptionally well at higher temperatures, it is unsuitable for structural applications such as high temperature engine components that must be repeatedly heated up through intermediate temperatures. Upon the repeated thermal cycling of such engines the MoSi.sub.2 components would simply pest and fall apart.
Pesting is a general term describing the catastrophic oxidation of intermetallic materials at intermediate temperatures. The accelerated oxidation leads to the disintegration or `pesting` of the material and the component falls apart. For MoSi.sub.2 the temperature at which pesting is most pronounced is approximately 773K. It has been observed that at 773K, bulk (i.e., non-composite) MoSi.sub.2, as well as composites of MoSi.sub.2 with alumina and aluminum nitride, all suffer total disintegration within 100 hours. It was concluded that the pesting of MoSi.sub.2 was little affected by the density of materials and the presence of foreign additives. The pested samples yielded powdery products consisting of MoO.sub.3 whiskers, SiO.sub.2 clusters, and residual MoSi.sub.2. The MoO.sub.3 whiskers exhibited protruding characteristics and were concentrated at grain boundaries and cracks. The pesting phenomenon in MoSi.sub.2 has been concluded to be the result of the formation of voluminous molybdenum oxides in microcracks. While not wanting to be bound by theory, the accelerated oxidation apparently involves the simultaneous formation of MoO.sub.3 and SiO.sub.2 in amounts essentially determined by the Mo and Si concentrations in the intermetallic.
In the last five years, an extensive amount of work has been carried out in efforts to overcome the problems associated with the use of MoSi.sub.2 as a high temperature structural material. While solid solution alloying, dispersion strengthening and fiber reinforcing have had limited success, these approaches still have not made MoSi.sub.2 a viable structural material.
Alloying with tungsten, tungsten disilicide or rhenium has improved high temperature creep strength, but not resolved the problems of pesting or matrix cracking. While substantial improvements in high temperature strength have been achieved by dispersing aluminum oxide in the matrix, again the pesting problem has not been resolved.
Known techniques of fiber reinforcement have been unsuccessful at curing this problem because MoSi.sub.2 has a relatively high coefficient of thermal expansion (CTE) as compared to most potential reinforcing materials, such as silicon carbide (SIC) fibers. The CTE mismatch between the fiber and the matrix material results in matrix cracking during fabrication and severe matrix cracking during thermal cycling, which of course leads to component failure. Candidate fibers include high strength ceramic fibers such as silicon carbide, single crystal alumina, and ductile, high strength molybdenum and tungsten alloy fibers. Ductile niobium fibers have been shown to improve the low temperature strength and toughness but a severe reaction occurring between the Nb fiber and MoSi.sub.2 matrix material limits its use and the high temperature characteristics were not improved at all. While the addition of silicon carbide (SIC) whiskers has yielded improvements in room temperature toughness, pesting and CTE mismatch are still a problem.
In spite of its high CTE, refractory metal fiber reinforcement of MoSi.sub.2 matrices was shown to increase both creep strength and toughness. The addition of about 40 vol % of low thermal expansion phase such as SiC in the form of whiskers and particles was shown to lower the thermal expansion of MoSi.sub.2 base matrices and reduce matrix cracking of refractory fiber reinforced composites. However, there was a severe reaction between the refractory fibers and the matrix. Matrix cracking was observed during consolidation with a silicon carbide fiber reinforced composite even with the matrix containing up to 40 vol % silicon carbide to modify the thermal expansion. An SCS-6 (silicon carbide)/MoSi.sub.2 -40 vol % SiC composite survived only 5 thermal cycles at 1573K and was completely destroyed within 100 hours of exposure to air at 773K.
Sapphire fiber reinforced composites showed no evidence of matrix cracking due to the good thermal expansion match between MoSi.sub.2 and sapphire, but because sapphire bonds strongly with MoSi.sub.2 it does not provide any improvement in the toughness.
While some form of reinforcement with continuous fibers having high strength and high aspect ratio appears necessary for the strength and damage tolerance required for high temperature aerospace applications, a solution to the pesting and matrix cracking problems has still not been found.
Improvements in fabrication of MoSi.sub.2 have led to materials having less porosity and correspondingly less susceptibility to pest attack. However, because of increased surface areas and complexities of fabrication from incorporating reinforcement phases in MoSi.sub.2 based composites, pesting of the composite material is still a major concern and a principle obstacle that must be overcome before the next generation aerospace goals can be achieved. In short, one of the most promising materials for such applications cannot be used unless a solution to the forgoing problems is found.